This invention relates to a thermoplastic polymeric composition which comprises a linear aromatic polyester of a bisphenol and a dicarboxylic acid in admixture with certain organic phosphate acid and organic phosphate salt compounds and which exhibts an enhancement of tensile strength on ageing. More particularly, the invention is directed to such compositions wherein the organic phosphate compound is a perfluoroalkenoxy-substituted surface active compound.
Linear aromatic polyesters prepared from dicarboxylic acids (especially aromatic dicarboxylic acids) and bisphenols are well known for their suitability for molding, extrusion, casting, and film-forming applications. For example, U.S. Pat. No. 3,216,970 to Conix, discloses linear aromatic polyesters prepared from isophthalic acid, terephthalic acid, and a bisphenolic compound. Such high molecular weight compositions are known to be useful in the preparation of various films and fibers. Further, these compositions, when molded into useful articles using conventional techniques, provide properties superior to articles molded from other linear polyester compositions. For instance, aromatic polyesters are known to have a variety of useful properties, such as good tensile, impact, and bending strengths, high thermal deformation and thermal decomposition temperatures, resistance to UV irradiation, good electrical properties, relatively good ease of processability and stability on processing.
Aromatic polyesters which are particularly well suited for molding applications may also be prepared by reacting an organic diacid halide with a difunctional aliphatic reactive modifier, such as a glycol, and subsequently reacting this product with a bisphenol compound. The resulting polyesters have reduced melt viscosities and melting points which permits molding at temperatures within the operable limits of conventional molding apparatus (i.e. less than about 300.degree. C.). This type of glycol-modified polyester is more fully disclosed in U.S. Pat. No. 3,471,441, to Hindersinn et al.
However, a persistent problem with aromatic polyesters has been found to be a pronounced tendency of the aromatic polyester to lose tensile strength on ageing, especially on ageing at elevated temperatures.
Thus, it is found that on ageing at 160.degree. for 60 days a bisphenol A-isophthalate-terephthalate polyester loses more than about 32% of its initial tensile strength.
The prior art does not disclose the use of additives to linear aromatic polyesters to enhance the tensile strength thereof on ageing. The prior art has proposed the addition to polyesters of various organic phosphate compounds and fluorinated organic compositions which are distinctive from the additive of the invention. Moreover, the use of the prior art additives produces modifications of the properties of the polyester other than the tensile strength enhancement on ageing achieved by the invention. For example Belgian Pat. No. 812,827 (to Teijin) published Mar. 28, 1973, discloses a composition comprising 1) an aromatic polyester of a glycol component comprising at least 70 mole percent tetramethylene glycol and at least 70 mole percent of an aromatic dicarboxylic acid (e.g. polybutylene terephthalate) and 2) 5 to 50% of a polycarbonate which contains 0.01 to 3% (based on the polyester) of a discoloration prevention agent which, inter alia, is a simple monoester of phosphoric acid or salt thereof such as the mono-methyl ester of phosphoric acid. It is noted that the reference composition is distinguished from that of invention since the composition contains a substantial amount of a polycarbonate which is known to be distinctive in properties from the dicarboxylic acid-derived polyesters of the invention. The additive of the reference is also distinguished from the additive copound of the invention which, as a surface active compound, contains in addition to the ionogenic phosphate group a highly hydrophobic and olephobic perfluoroalkenoxy group. Furthermore the additive of the invention, unlike that of reference does not prevent polymer discoloration (as described in Example 2 below). In any case, prevention of polymer discoloration is not suggestive of the beneficial effect of the present additive, namely tensile strength enhancement on ageing.
U.S. Pat. No. 4,002,581 (T. J. Dolce to General Electric Co.) discloses a foamed composition comprising a linear polyester such as polybutylene terephthalate, a polymer of an alpha olefin and a polytetrafluoroethylene resin having improved toughness. The reference compositions may contain a minor proportion of a flame retardant, inter alia, a phosphate ester. However, the phosphate esters disclosed by the reference are di- or tri-esterified phosphoric acid devoid of the ionogenic group and hydrophobic/olephobic group present in the surface active compound additive of the invention. The flame retardant property achieved by use of the reference phosphate additive is also not suggestive of the tensile strength enhancement property of the present compositions. Furthermore, the fluorine-containing resin additive of the reference is distinguished from the present additive in being a thermoplastic resin containing only fluorine substituents whereas in contrast the present additive is a surface active compound substituted with additional groups such as --SO.sub.2 -- and hydrogen.
U.S. Pat. No. 3,644,593 (B. E. Nowak et al. to Carborundum Co.) discloses mixtures of an oxybenzoyl polyester and a polyfluorinated addition polymer of a fluorinated monomer (such as tetrafluoroethylene and vinylidene fluoride) which have one or more of the following properties: better lubricant property, less brittleness, better wear resistance and low creep. The fluorinated resin additive of the reference is structurally distinguished from the additive of the invention in being a resin devoid of the ionogenic substituent present in the additive of the invention. Moreover, none of the enhanced properties ascribed to the reference composition is suggestive of the enhancement, on ageing, of the polyester tensile strength which is achieved by the invention.
U.S. Pat. No. 3,880,798 (W. R. Deem et al. to Imperial Chemical Industries Ltd.) describes use of surface active agents (inter alia the surface active compound of the present invention) as mold release agents in molding polymers such as polyolefins and polyurethanes. Use of a mold release agent generally involves addition of the mold release agent to the mold and does not involve mixing the mold release agent with the polymer as taught by the invention. Moreover, the reference polymers for which use of the surface active agent as mold release agent is prescribed are structurally remote from the present polyesters.
Accordingly, it is a principal object of the present invention to provide linear aromatic polyester compositions which have enhanced tensile strength on ageing and yet substantially retain the other desirable properties of the polyester such as ease of processing and thermal stability on processing.